Abstract: The fracture causes of a tubing was studied through analysis of fracture characteristics, material properties and load, combined with service environment and operation conditions. The results show that the sulfur content in oil tubing exceeded the standard, but content of other elements, microstructure and mechanical properties of the fractured tubing met the standard requirements. It was found that the fracture was characterized by ductile fracture without obvious corrosion from the analysis of fracture characteristics. According to the load calculation, when there was a leakage point in the casing near the fractured tubing, a pressure difference between the annulus and the formation formed. The tubing contacted with the casing under the action of pressure difference, and was worn constantly by moving up and down under the action of additional stress. Finally it was perforated and fractured.